The present invention relates to a measuring system with ratiometric frequency output and a method for improving the measurement accuracy of such a system.
Measuring systems having one or several sensors and an associated evaluation unit are widespread. To illustrate the principal design of measuring systems of this type, an air-mass sensor that is known from automotive technology will be described hereinbelow in greater detail as an example. The invention should not be limited to air-mass sensors, however.
In order to determine the flow of a flowing medium, such as air, fuel-injection installations include a mass flow meter, which is also referred to in the literature as an air-mass sensor. With known mass-flow sensors of this type, the sensor element is exposed to a stream of air in the intake manifold of the internal combustion engine. The sensor element has a heater and measuring resistors that are cooled via convection by the air stream, which brings about a change in resistance. The air stream flowing through the intake manifold can be determined from the unbalance of a measurement bridge. Finally, the sensor provides a measured signal, which is transmitted to a distant evaluation unit.
For this purpose, an air-mass sensor further includes a (digital) interface for transmission of the measured signal. The evaluation unit extracts the useful information from the received signal and evaluates it.
A typical example of a measuring system of this type having a digital interface is shown in a schematic representation in FIG. 3. The measuring system shown includes a measuring device 1 having an interface 5 for the transmission of digital signals to an evaluation unit 2. Measuring device 1 and evaluation unit 2 are interconnected via a cable 11.
Interface 5 is based substantially on digital circuit technology and includes a clock input 3, to which a clock pulse having a certain frequency (e.g., 10 MHz) is supplied, and a signal input 10, to which the measured signal from the sensor is applied.
Interface 5 further includes an arithmetic unit 6, which processes the measured signal and outputs a corresponding signal at signal output 7 of interface 5.
The output signal is typically a signal derived from the system clock signal and the measured signal. As a result, a linear relationship usually exists between the clock signal and the output signal.
Transmission of the useful information (the measured value) can be carried out basically using all known transmission methods, such as modulation procedures. The useful information can also be depicted in the on/off ratio or in the frequency and/or period duration of the output signal.
With known systems, the useful signal is usually contained in the period duration of the output signal, since a coding of this type is relatively easy to realize and it enables very high measurement accuracy.
The system clock signal that is applied to clock input 3 is generated by a pulse generator 4, such as an oscillator or quartz. Oscillators or quartzes of this type can have high tolerances and/or pulse fluctuations. The deviation of the system clock signal affects the output signal in a directly proportional manner, however, and can therefore strongly interfere with measurement accuracy.
With applications having a large measuring range, in particular, measuring systems of this type can fulfill the specified requirements for measurement tolerance only if highly-accurate quartzes and/or oscillators are used. Precise quartzes are correspondingly expensive, however, and they cannot be integrated directly, due to reasons of cost.